18/6 vs 24/0: The Science

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Okay, so I know this argument has had to death, but what I've noticed is that there's always this idea of, "Well, having a dark period is more natural for a plant, so that's better." Despite the fact that there are places in the globe where plants do get 24hrs (or near that) of light, and that these areas are known for growing giant vegetables and dank weed (Matanuska ftw), they still don't seem to think that 24 hrs of continuous light is "healthy" for a plant. Beyond that, the idea of a 18/6 schedule being more "natural" is a little hard to swallow as well, as not many places on the earth receive 18 hours of light.

So, scientifically speaking, this argument gets confounded by the fact that there's something called "dark reactions" in botany. The problem with this descriptive name is people get the idea that these reactions only occur during night, and that a night period is somehow needed or they won't occur. A more apt descriptor is now adopted called "light independent reactions" for the "dark reactions"; you should Google this on your own if you wish. Long story short, the plant changes the way it produces ATP from photosynthesis, to the Calvin-Benson cycle. If you want to learn more about these, consult any botany article

The main thing at work here is again the idea that these processes are usually called "dark reactions", but these "dark reactions" can and do happen during the daylight as well. However, there is a little bit of a caveat with "can plants recieve 24 hours of light," and that is a little difference in the Calvin-Benson cycle. C3 and C4 plants can grow normally in the dark, but CAM plants need darkness at some point to convert ATP into glucose. C3 and C4 plants can do this whenever.

So with all that in mind, I have to say that I've found absolutely no evidence to suggest that plants need a "rest" or "dark" period and nothing to suggest that they cannot grow just fine in 24 hours of light.

To recap, "dark reactions" is a misnomer, and there is nothing that C3 and C4 plants do in the dark that they cannot do in the light.

Active Member

Okay, so I know this argument has had to death, but what I've noticed is that there's always this idea of, "Well, having a dark period is more natural for a plant, so that's better." Despite the fact that there are places in the globe where plants do get 24hrs (or near that) of light, and that these areas are known for growing giant vegetables and dank weed (Matanuska ftw), they still don't seem to think that 24 hrs of continuous light is "healthy" for a plant. Beyond that, the idea of a 18/6 schedule being more "natural" is a little hard to swallow as well, as not many places on the earth receive 18 hours of light.

So, scientifically speaking, this argument gets confounded by the fact that there's something called "dark reactions" in botany. The problem with this descriptive name is people get the idea that these reactions only occur during night, and that a night period is somehow needed or they won't occur. A more apt descriptor is now adopted called "cellular respiration" for the "dark reactions", and you can Google this on your own if you wish but long story short, the plant changes the way it produces ATP from photosynthesis, to cellular respiration. If you want to learn more about these, consult any botany article.

The main thing at work here is again the idea that these processes are usually called "dark reactions", but these "dark reactions" can and do happen during the daylight as well. However, there is a little bit of a caveat with "can plants recieve 24 hours of light," and that is a little difference in the Calvin-Benson cycle. C3 and C4 plants can grow normally in the dark, but CAM plants need darkness at some point to convert ATP into glucose. C3 and C4 plants can do this whenever.

So with all that in mind, I have to say that I've found absolutely no evidence to suggest that plants need a "rest" or "dark" period and nothing to suggest that they cannot grow just fine in 24 hours of light.

To recap, "dark reactions" is a misnomer, and there is nothing that C3 and C4 plants do in the dark that they cannot do in the light.

Well-Known Member

Well, first of all, cannabis is considered a C4, at least according to "Marijuana Botany" by Robert Conell Clarke. It's one of the oldest references of botanical data for cannabis, so it may be out of date information, but I doubt it as cannabis is one of those plants that doesn't need/like all that much water in comparison to others.

So C3, C4 and CAM refer to something called Carbon Fixation, and this is a process which usually involves CO2 entering the leaf from the stomata ( little air holes on the bottoms of the leaves ), but the problem with this approach is that the stomata also allow the outside air to dehydrate and suck moisture from the plant. The key difference between C3 and C4 varieties is a little nuance in what is called the Calvin-Benson cycle, which is basically the biomolecular cycle's name and quite a lot to explain without charts and diagrams and everything so I'll just outline the practical differences...

C4 plants use about 25% less water than C3 plants do
CAM plants use about 75% less water than even C4 plants, but NEED a dark period.

The reason CAM plants need a dark period is because their strategy is to wait until night to open their stomata and fixate carbon.

C4 plants are usually things like grasses that grow in arrid and high temperature conditions, and CAM plants are succulents and desert dwelling plants.

Here's a video that I thought was really good at explaining the really nitty-gritty differences, but it might not make much since to you without first looking up a bit about photosynthesis, cellular respiration and the "photosynthetic equation". But I mean I was really impressed with the level of detail and information, so I'm gonna link it up just for the sake of accuracy. I'll also freely admit I don't know this stuff like the back of my hand.

Yes this is quite a hefty subject. I've been wrapping my head around it and am even taking a class on it...

About the Emerson Effect, I'll be honest this is the first I've ever heard about it. That sounds very interesting and I'm going to look into it! I think it might be a little outside of the scope for this discussion, my gut reaction is that it must not produce yields significantly higher to compensate for the cost of the technology. In other words, maybe it produces a 15% increase in yield, but costs 100% more in lighting (since we're essentially talking about two separate lights). Briefly Google'd it and saw there are some commercial offerings of lights with both IR and FR lights in the same device, but again they still seem quite costly so I'm not sure how much experimentation people have been doing or if it's something economically viable. However, everything that I can see about it so far seems to suggest it's a 100% verified phenomenon and even helped them discover the two distinct photosynthesis systems.

I realized one of the reasons the C3 vs C4 distinction might come into play is photorespiration. That's when the stomata on a C3 plant close, and O2 begins to build up and deplete the CO2. The RuBG that binds with CO2 also binds with O2, but it produces CO2 by breaking down glucose and using ATP. The problem is it doesn't produce any more ATP, so this just ends up using sugar and ATP, and that's very bad for a plant.

I've seen some people with the misconception that the stomata will only open at night, and so they may be under the impression that the plant is suffering this photorespiration through the entire day-light cycle, and that the dark period might give it some kind of "reprieve". I've also heard the idea that it's at night that it builds up some kind of energy reserves. Neither one really seems to hold much truth, and is rather a hodgpodge of half-truths confused with misconceptions. The plant will close its stomata to avoid dehydration, but not at the cost of killing itself with photorespiration. As for the idea that its building up energy reserves, it could do that far better with photosynthesis, which it needs light to do.

The reason I don't see this as a problem even for C3 plants is that if your temperature and humidity are controlled, then the plant will not spend much time with its stomata closed whether it is light or dark. Of course, the caveat here is that extended light times may build up more levels of heat which will result in the closing of stomata, but that is more about environmental control than it is about plant physiology. If a small dark period allows the temperatures to drop below 30 C, then it is certainly better than keeping them on beyond 30 C. If you have an environment where heat is uncontrolled that might be something to think about, but again the trade-off is the time spent photosynthesizing you might lose.

New Member

My college biology text used the term "dark cycle," but the professor for the course refused to use the term for just that reason I think--it's misleading. Plants respire (i.e. use O2) all the time just like us, dark or light. The "dark" part of the name seems to be to indicate that the reactions don't require light, and not/I] that they take place in darkness.

As for 24/0: I grew an auto with continuous light for seven weeks and had a huge harvest, so as a data point, one auto in one grow did great with 24 hour light. It would be interesting to see how a photoperiod plant did with 24 hour light. Hmmm... I'm starting a new grow--sounds like a fun experiment...

Well-Known Member

My college biology text used the term "dark cycle," but the professor for the course refused to use the term for just that reason I think--it's misleading. Plants respire (i.e. use O2) all the time just like us, dark or light. The "dark" part of the name seems to be to indicate that the reactions don't require light, and not/I] that they take place in darkness.

As for 24/0: I grew an auto with continuous light for seven weeks and had a huge harvest, so as a data point, one auto in one grow did great with 24 hour light. It would be interesting to see how a photoperiod plant did with 24 hour light. Hmmm... I'm starting a new grow--sounds like a fun experiment...

Well, in my experience photoperiod plants vegetate under 24 hours of light quite well, but of course they cannot flower with that length of day. It would kind of stand to reason that autoflower plants must be able to build their flowers faster since they can photosynthesize for 24 hours and still photosynthesize, but I don't know if that theory really carries over in practice because I've never heard of people getting higher yields off auto plants. The caveat to that is they usually vegetate a photoperiod plant to a huge monster size that's far beyond what the auto achieves before flowering, so I think it would stand to reason that if you put a flowering photoperiod plant into a 12/12 cycle when it was the same size as a similar auto plant that was going into flower, that the auto plant would grow faster. Of course because of the genetic differences there's no real like-strains to really compare like that.

One thing I think demonstrates this idea though is 12/12 from seed with photoperiod plants. They typically begin to flower in about 3-4 weeks, and from what I hear that's about similar to when auto flowers start. However, if you look at the yields people get off of photoperiod plants that remain in 12/12 versus what people get off of autoflower plants, it would seem that the autoflower's ability to photosynthesize for 24 hrs a day is going to mean both bigger vegetative stature and bigger yield even though both plants will be able to flower when they're mature.

It would be interesting to try to take a clone off of a large photoperiod plant and root it, get it into flower, and have it be the same size as an auto version of the same strain when it started flowering. Then have one in 24 hrs flowering, and the photoperiod in 12 hrs. Considering the ruderalis traits in autos, if it kept up in size or overtook the photoperiod plant then you could be pretty certain it is because of the extra 12 hours of photosynthesis available to it.

Well-Known Member

I know it is not scientific info, but I am currently wading through The Cannabis Grow Bible 3rd edition for a review & the author surprised me by his strong insistence on 24/0 for veg throughout the book. He acknowledges 18/6 but feels that unless you really need the energy savings, you are just reducing your potential yield.

Active Member

Well, in my experience photoperiod plants vegetate under 24 hours of light quite well, but of course they cannot flower with that length of day. It would kind of stand to reason that autoflower plants must be able to build their flowers faster since they can photosynthesize for 24 hours and still photosynthesize, but I don't know if that theory really carries over in practice because I've never heard of people getting higher yields off auto plants. The caveat to that is they usually vegetate a photoperiod plant to a huge monster size that's far beyond what the auto achieves before flowering, so I think it would stand to reason that if you put a flowering photoperiod plant into a 12/12 cycle when it was the same size as a similar auto plant that was going into flower, that the auto plant would grow faster. Of course because of the genetic differences there's no real like-strains to really compare like that.

One thing I think demonstrates this idea though is 12/12 from seed with photoperiod plants. They typically begin to flower in about 3-4 weeks, and from what I hear that's about similar to when auto flowers start. However, if you look at the yields people get off of photoperiod plants that remain in 12/12 versus what people get off of autoflower plants, it would seem that the autoflower's ability to photosynthesize for 24 hrs a day is going to mean both bigger vegetative stature and bigger yield even though both plants will be able to flower when they're mature.

It would be interesting to try to take a clone off of a large photoperiod plant and root it, get it into flower, and have it be the same size as an auto version of the same strain when it started flowering. Then have one in 24 hrs flowering, and the photoperiod in 12 hrs. Considering the ruderalis traits in autos, if it kept up in size or overtook the photoperiod plant then you could be pretty certain it is because of the extra 12 hours of photosynthesis available to it.

Active Member

Old thread I know but I'm really curious about 24/0 light cycle. I have not found any studies to say anything either way just growers who "feel" the plant needs rest. That doesn't hold any water in my book without science to back it up or empirical data. I have seen a few side by sides 18/6 vs 24/0 that reflect better growth by a 24 hr light cycle...wish I could find something more definitive.

New Member

I wrote to a professor of biology about this and got the following response...

There will be some upper limit on how much carbon a plant can fix (i.e., how much sugar it can produce) based on limiting factors other than light exposure. These limiting factors could include key nutrients (such as phosphorus needed to make ADP and ATP in the first place) or water. I would expect even under ideal conditions the intermediary biochemical pathways that intersect with cellular respiration and photosynthesis will have some upper rate (that is, the kinetics of the reactions involved). So, in general, more exposure to light and thus more energy captured through the light reactions would enable more carbon fixation by the dark reactions, but an asymptote will likely be reached at some point.

Another factor to consider is that particular plant species are adapted to different light conditions and species that have evolved in low light conditions (such as in a rainforest understory) would probably be disadvantaged by exposure to too much light.

There is only one way that this question can be answered once and for all time. You simply have to set up two identical grows with two identical clones and see for yourself. Unless you do that or know someone who has done that, it is a debate that never ends.

New Member

Old thread I know but I'm really curious about 24/0 light cycle. I have not found any studies to say anything either way just growers who "feel" the plant needs rest. That doesn't hold any water in my book without science to back it up or empirical data. I have seen a few side by sides 18/6 vs 24/0 that reflect better growth by a 24 hr light cycle...wish I could find something more definitive.

Well-Known Member

I know this has always been a long time debate... my personal experience has never to leave the lights on for 20/4 for the entire veg cycle yet I start at 20/4 once I have good roots established and after a couple weeks I start to dial the timer back 2 hours every 2 weeks until I’m at 12/12, I have always seen good results from this and I run (4) “600” watt leds, my last grow yielded a few ounces shy of my 2 pound goal with 8 plants, 2 plants were lacking bud production big time and I feel if I had 8 clones from my best pheno in that room 2 pounds would have been easily achievable especially under optimal conditions, as it was I was not controlling air temp or water temp just simply adding a little Clorox to keep root issues at bay and seemed to work fine. I can’t say 20 hrs light would have helped these girls as some already were getting to overgrow my room even after being scrogged. It’s a matter of messing with things, after doing it for a while you’ll get a feel for what your plant wants, but all strains each have their own variations and requirements soo it will constantly be a learning curve for everyone unless you stick with one strain and run a grow from clones from one mother, different variables will always need to be met

Grow Journal of the Month: Dec 2017

I just ran a clone that was cut, exposure to roots was 24/0. Once there was roots on the clone I put it directly into soil and into 12/12 nothing but water and soil.

The clone is flowering now next to its mother (clone of a clone). The clone I went direct to 12/12 started out with 5 fingered fan leaves the fans she had at cutting, and sometime after the transplant flip, the plant went to all single leaf fan leaves. She continuned and is still that way now @ 96 days in flower. Still green (her mother is now almost all gold), stunted growth very short. Flowers from soil line up to tops several branches.

She reminds me of a re-veg plant that never took off.

Here's a pic from 3 weeks ago. She's growing very slow. Seems like her metabolism is slowed down significantly.

DT Mutant @ 98 days ... looks the same as 60 days. Not sure how long this one can go but looks...

Here's her mother - she's only been in FLOWER for 49ish days and has another 60 to go (Landrace variety) - Plants turn gold once stretch in over @ ~35-45 days in flower. The mutant clone is still VERY green. I've grown this plant out from seed, selected 2 Phenotypes. This one here is the longest flowering version.

Member

I've always run my lights 24/0 and have great results. I thought about switching to 18/6 for saving money on electric but worry about having to veg longer? Either way I think if it works for you don't change it.
Nice looking plants bobrown14

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